Imagesetters and platesetters are used to expose the substrates that are used in many conventional offset printing systems. Imagesetters are typically used to expose the film that is then used to make the plates for the printing system. Platesetters are used to directly expose the plates.
In the case of platesetters, for example, the plates are typically large substrates that have been coated with photosensitive or thermally-sensitive material layers, referred to as the emulsion. For large run applications, the plates are fabricated from aluminum, although organic plates, such as polyester or paper plates, are also available for smaller runs.
Computer-to-plate printing systems are used to render digitally stored print content onto these printing plates. Typically, a computer system is used to drive an imaging engine of the platesetter.
The imaging engine selectively exposes the emulsion that is coated on the plates. After this exposure, the emulsion is developed so that during the printing process, ink will selectively adhere to the plate""s surface to transfer the ink to a print medium.
In imagesetters and platesetters, throughput is a critical metric. These systems typically operate in commercial environments. Their throughput is often used as the criteria for selecting between the various commercially available systems.
The cycle time, and consequently throughput, for a platesetter or imagesetter is dictated by the time required: 1) to load the substrate into the imaging engine; 2) for the engine""s scanner to expose the substrate; and 3) to unload the substrate. Most conventional systems expose the media by scanning. In a common implementation, the plate or film media is fixed to the outside or inside of a drum and then scanned with a laser source in a raster fashion. The laser""s dot is moved longitudinally along the drum""s axis, while the drum is rotated under the dot. As a result, by modulating the laser, the substrate is selectively exposed in a continuous helical scan.
The typical approach to reducing the cycle time of the imaging engine focuses on decreasing the time required for the engine""s scanner to expose the substrate. Some have approached this problem by increasing the speed at which the laser is modulated, enabling the drum to be rotated at a higher rate. There are limitations, however, in the laser""s power and its speed of modulation. Other solutions use spatial light modulators, so that multiple lines of the image can be exposed in each rotation of the drum.
An alternative path to decreasing cycle time concerns loading multiple substrates simultaneously on the drum. In one example, a number of substrates are positioned along the drum""s axis. In still another approach, multiple substrates are loaded around the drums"" circumference.
This, however, tends to have a limited impact on cycle time. The exposure step is consequently longer, since more substrate surface area must now be exposed. Further, the time to load and unload is also not substantially affected since multiple substrates cannot be loaded on the drum simultaneously.
The present invention is directed to decreasing the cycle time and thereby increasing throughput in a substrate exposure system, such as a platesetter or imagesetter. It decreases cycle time by optimizing the loading and unloading of substrates.
Specifically, according to the invention, substrates are loaded and unloaded on the same drum simultaneously. As a result, in some implementations, the time to unload and load a single substrate can be reduced by half. That is, the time to unload any given substrate is amortized over the time to load a subsequent substrate.
In general, according to one aspect, the invention features a method for loading and unloading substrates on a drum of an imaging engine.
In the present embodiment, the substrates are plates and the imaging engine is used in a platesetter.
The method comprises unclamping a trailing edge of a first substrate from the drum and clamping a header of a second substrate to the drum. The order in which these two steps are performed is not critical to the invention. For example, the clamping and unclamping can happen simultaneously. Alternatively, the unclamping of the trailing edge can occur before the clamping of the header, or visa versa.
In any case, the drum is then rotated to eject the first substrate from the drum while installing the second substrate on the drum. The notion here is that, as the drum is rotated, one substrate is being ejected while a second substrate is being installed on the drum. Thereafter, the header of the first substrate is unclamped and the trailing edge of the second substrate is clamped to the drum.
Here again, the order in which these final unclamping and clamping steps is performed is not critical. They can occur simultaneously. In other examples, the unclamping occurs before the clamping, or visa versa.
In specific embodiments, the step of unclamping the trailing edge of the first substrate comprises removing a first removable clamp from the drum.
Further, the step of unclamping the header of the first substrate comprises opening a first fixed clamp on the drum, and the step of clamping the header of the second substrate comprises holding the header of the second substrate to the drum with a second fixed clamp.
In the preferred embodiment, the method further comprises loading the first substrate on the drum by clamping the header of the first substrate, and then rotating the drum in a first direction to install the first substrate on the drum and then clamping the trailing edge of the first substrate. This allows this first substrate to then be ejected as previously described.
In the preferred embodiment, the step of rotating the drum to eject the first substrate while installing the second substrate comprises rotating the drum in a second direction, which is opposite the first direction, i.e., the direction in which the drum is rotated when the first substrate is initially installed. As a result, in sequential loading/unloading steps, the drum is rotated in opposite directions.
In general, according to another aspect, the invention comprises an external drum imaging engine. Specifically, it comprises a drum on which substrates are installed and a scanner for exposing the substrates that are installed on the drum. Two fixed header clamps are provided. They are positioned at different locations around the perimeter of the drum for clamping headers of different substrates to the drum. At least one trailing edge clamp is provided for clamping trailing edges of substrates to the drum.
In the preferred embodiment, the header clamps are only operational on alternating substrate exposure steps. Specifically, during any exposing step, only one of the header clamps is operational, i.e., clamping a header of a substrate, while the other header clamp is non-operational.
Preferably, two input/output ports are provided. Each port either supplies or receives a substrate while the other port receives or supplies, respectively, a different substrate.
The trailing edge clamp is preferably a movable clamp and is, therefore, adjustable in its clamping location around the circumference of the drum. In some examples, two trailing edge clamps are provided and are used during alternating exposure steps, i.e., the two trailing edge clamps are never clamping substrates to the drum at the same time.